A digital memory is arranged as an array of individual memory cells, each cell representing a single bit. This invention is an improvement in the structure of a particular type of digital memory cell: the bipolar static flip flop.
The basic bipolar memory cell is well known. It consists of a pair of bipolar transistors cross-coupled to form a flip flop. The base of one is directly connected to the collector of the other and the collector of the other is directly connected to the base of the first. The flip flop, or cell, is completed by providing a suitable loan means connected to each collector, and power supply voltages.
When the basic bipolar memory cell is formed with a buried layer collector, a special problem arises in contacting the buried layer. Normally, the base region essentially covers the buried collector. In order to provide the direct, or ohmic, connection to the buried layer, it is necessary to extend the buried layer collector so that a portion of it protrudes outwardly in the horizontal plane. The extended portion of the buried layer collector has no base layer above it. Therefore, the buried collector may be extended to the planar surface of the integrated circuit. Once the buried layer is accessible on the surface, a direct connection is used to cross-couple the memory cell. Cross coupling the cell as described above often requires special processing steps and increases the size of the basic cell.